1. Field of the Invention
The present invention relates to methods and apparatus for chemical vapor deposition onto a substrate, and more particularly to a method that deposits silicon at a high rate due to enhanced mass transport by thermal diffusion, i.e., the xe2x80x9cSoret effectxe2x80x9d, by using a temperature gradient above the substrate surface.
2. Description of the Prior Art
The semiconductor industry has been depositing poly crystalline silicon for a number of years. The method of choice for most applications is a Low Pressure Chemical Vapor Deposition (LPCVD) process. The LPCVD process is a well studied art wherein poly crystalline silicon deposition is accomplished by placing a substrate in a vacuum chamber, heating the substrate and introducing silane or any similar precursor such as disilane, dichlorosilane, silicon tetrachloride and the like, with or without other gases. The reactant gases are usually pre-heated prior to passing over a wafer when a rapid deposition is required. The pre-heating pre-activates the reactants and increases the rate of subsequent deposition. A disadvantage of this process is that it causes gas reactions that deplete the supply of available reactants which partially defeats the effect of pre-activation in increasing the deposition rate. Deposition rates of approximately 10 to 100 angstroms per minute are typical for low-pressure processes (less than 1 Torr) in a hot wall low pressure reactor. Deposition rates of 20 to 300 angstroms per minute are achieved in a vertical flow reactor with deposition rates as high as 500 angstroms per minute. Silicon deposition rates over 10,000 angstroms per minute have been reported, however these high deposition rates do not produce poly crystalline silicon films that are useful in manufacturing semiconductor devices because the resulting poly crystalline silicon has undesirable features such as large grain size, non uniform thickness, etc. Deposition rates of approximately 3000 angstroms per minute of useful semiconductor quality poly crystalline silicon are achieved with a higher pressure process (25 to 350 Torr) as described in detail in U.S. Pat. No. 5,607,724.
A typical prior art CVD system is illustrated in FIG. 1, and includes a reaction chamber 12 having a quartz tube 14. The chamber is enclosed on a first end by a seal plate 16 that can be removed for installation and removal of a boat 18 carrying substrates 20. A reactant gas 22 such as silane or similar precursor and hydrogen and a dopant gas such as phosphine are supplied to the chamber 12 through ports 24 and 25 to tubing 27 and flow through the chamber 12 and exit the exhaust port 26. A plurality of heater elements 28 are separately controlled and adjustable to compensate for the well known depletion of feed gas concentration as the gas 22 flows from the gas injection tube 27 to the exhaust port 26. The system of FIG. 1 typically operates at a gas chamber pressure in the range from 100 to 200 mTorr, and at a gas flow rate from 100-200 sccm. The reactant gas is usually silane diluted with hydrogen. Operating in the low pressure range of 100-200 mTorr with silane, or other similar precursor, results in a low deposition rate, typically in the range of 10 to 100 angstroms per minute, and 5 to 30 angstroms per minute if a dopant gas is introduced. The resulting surface roughness is typically 10-15 nm. Operation at higher concentrations of the reactant gases results in non-uniform deposition across the substrates, as well as large differences in the deposition rate from substrate to substrate. Increasing the gas flow rate in the chamber of FIG. 1 can improve deposition uniformity at higher pressures, but has the disadvantage of increasing the gas pressure resulting in gas phase nucleation causing particles to be deposited on the substrate.
There are other problems associated with the reactor of FIG. 1, such as film deposition on the interior surfaces of the quartz tube 14 causing a decrease in the partial pressure of the reactive feed gas concentration near the substrate surface. This results in a reduced deposition rate and potential contamination due to film deposited on the wall of tube 14 flaking off and falling on the substrate 20 surfaces. Another problem occurs due to the introduction of a temperature gradient applied between the injector end and exhaust end of the tube to compensate for the depletion of reactive chemical species from the entrance to the exit. As a result of this temperature gradient, the deposited poly crystalline silicon grain size varies from substrate to substrate, i.e. across the load zone, because the grain size is temperature dependent. This variation in grain size from substrate to substrate can result in variations of poly crystalline silicon resistivity and difficulties with the subsequent patterning of the poly crystalline silicon resulting in variations in the electrical performance of the integrated circuits produced.
A prior art vertical flow reactor 30 is illustrated in FIG. 2. This reactor is capable of deposition rates as high as 500 angstroms per minute. The substrates 32 are placed in a substrate carrier 34 in the reactor 30. The reactor chamber 36 is confined by a quartz bell jar 38 and a seal plate 40. The bell jar 38 is surrounded by a heater 42 for heating the substrates 32 to the required temperature. The reactant gases such as silane and hydrogen are introduced through ports 44 and 46, and flow through the gas injection tube 48 to the injector 50, across the substrate 32 and out the exhaust port 52. The arrangement of FIG. 2 greatly reduces the gas depletion effect experienced with the device of FIG. 1, and thereby allows an increased gas flow which results in an increased deposition rate of up to 500 angstroms per minute. Two major problems are associated with the apparatus of FIG. 2. In operation, the injection tube 48 and injector 50 are at the same temperature as the substrates 32, a condition that results in silicon deposition in and on the injection tube 48 and injector 50, which then flakes off and is deposited as particles on the substrate 32. The other major problem is that the substrates 32 are not at the same temperature due to the method of heating the substrates from heater 42 with no heater below the substrates. The non-uniform heating causes a non-uniform silicon deposition over the substrates 32 as poly crystalline silicon deposition is a surface reaction rate limited process, which is very temperature dependent.
FIG. 3 shows a prior art single substrate reactor 54 that overcomes some of the problems associated with the reactors of FIGS. 1 and 2. This is described in detail in U.S. Pat. No. 5,607,724. A substrate 56 is placed on a rotating pedestal 58 in chamber 54. Upper lamps 62 and lower lamps 63 radiate energy through transparent chamber walls 64 and 66 to uniformly heat the substrate 56. The pedestal 58 is turned to rotate the substrate 56, which is heated on both sides by the lamps. The substrate temperature is therefore uniform over its surface, which results in a uniform poly crystalline silicon deposition on the substrate 56. The reactor 54 does not have an injector in the chamber, which eliminates the problem of deposition on the injector 50 shown in FIG. 2. The reactant gas 67 is supplied through an inlet port 68 and exits exhaust port 70. The major problem associated with the reactor of FIG. 3 is the limited throughput, i.e. the number of substrates processed per hour. This problem can be addressed by increasing the operating pressure to 10 Torr or greater resulting in high deposition rates exceeding 1000 angstroms per minute, however operating the reactor at such high pressures can result in a gas phase reaction where silicon particles are formed in the gas and deposited on the substrate. Another problem associated with the reactor is the tendency for silicon deposition on the quartz walls 64, 66 resulting in loss of radiant energy transmission from the lamps 62, 63. This causes non-uniform heating of the substrate resulting in non-uniform film deposition on the substrate 56. Additionally, the silicon deposited on quartz wall 64 can flake off and fall onto the substrate surface 56.
Current demands of semiconductor processing require rapid film deposition with uniform and repeatable film thickness, and the smoothest film surface possible with controlled grain size. In addition, the time the substrate is above 600xc2x0 C. must be held to a minimum, as heating the substrate to elevated temperatures, i.e. greater than 600xc2x0 C. results in unwanted diffusion of dopants. Because of this, a high deposition rate is important to reduce the time that the substrate is above 600xc2x0 C. Good film uniformity and repeatability is necessary to ensure consistent electrical performance, and smooth films are required for sub-micron lithography processes.
It is therefore an object of the present invention to provide a method and apparatus for the Chemical Vapor Deposition (CVD) of various materials at a high rate.
It is a further object of the present invention to provide a method and apparatus for the CVD of various materials at a high deposition rate with improved uniformity.
It is another object of the present invention to provide a method and apparatus for the CVD of various materials at a high rate, with improved uniformity and reduced surface roughness.
Briefly, a preferred embodiment of the present invention includes a method wherein a substrate is placed in a reaction chamber and rotated to ensure uniform heating and a uniform flow of reactant gases over the substrate surface. Upper lamps positioned above the substrate and lower lamps below the substrate are activated to apply heat to an upper thermal plate and a lower thermal plate which in turn heat the wafer upper and lower surfaces. The upper and lower lamps are operated to raise the substrate temperature to 500-700xc2x0 C. for silicon deposition, or any other temperature required for the deposition of other materials, and to provide a heat gradient between the upper and lower thermal plates and thus cause a thermal gradient between the upper substrate deposition surface and the upper thermal plate. This heat gradient causes a large increase in the deposition rate for a given reactant gas flow rate and chamber pressure. The preferred parameters for implementation of the present invention include the temperature of the upper thermal plate adjusted to be 100-200xc2x0 C. above the temperature of the lower thermal plate, the substrate temperature in the range of 400-700xc2x0 C., the reactant gas pressure between 250 and 1000 mTorr, and the gas flow rate in the range of 200-800 sccm. The substrate rotation is approximately 5 RPM, however the speed of rotation is not critical. For example, a deposition rate of about 2000 angstroms per minute is achieved with a 100-200xc2x0 C. temperature differential between the thermal plates, substrate temperature about 650xc2x0 C., pressure of 250 mTorr and silane flow of 500 sccm.
An advantage of the present invention is that it provides a higher deposition rate CVD method with good film quality.
A further advantage of the present invention is that it provides a CVD deposition method with a deposition rate five times more rapid than prior art methods providing comparable film quality.